1. Field of the Invention
The present invention relates to an optical disc apparatus, in particular, an optical disc apparatus capable of reading/recording data from/on a plurality of types of optical discs such as DVDs (Digital Versatile Disc) and CDs (Compact Disc), which vary in specification.
2. Description of the Related Art
Optical disc apparatuses such as DVD players and DVD recorders are configured to be capable of reading/recording data from/on a plurality of types of optical discs such as CDs, CD-Rs and CD-RWs (hereinafter referred to as “CD” collectively) as well as DVDs, DVD-Rs, DVD-RWs and DVD-RAMs, which vary in specification.
Hereupon, wavelengths of the laster beams used for reading/recording of data in the DVD and CD are different from each other due to difference of their recording density. Thus, two semiconductor lasers (including an integrated laser chip) are used to, output two laser beams each having a different wavelength. On the other band, to reduce an optical pickup used in the optical disc apparatuses in size and weight and lower the costs, it has been proposed that laser beams outputted from the two semiconductor lasers are converged on an optical disc using the same optical system.
Furthermore, in these optical disc apparatuses, in order to detect a tracking error, the laser beam is separated into three beams of 0th-order diffracted light, and +1st-order diffracted light and −1st-order diffracted light by using a diffraction grating.
In Japanese Laid-Open Patent Publication No. 2002-190133, a diffraction grating for the DVD and a diffraction grating for the CD each are formed on both surfaces of one plate-like optical element, respectively. In such a method, since the angle which a grating groove of one diffraction grating forms with a grating groove of the other diffraction grating is a smaller as 0.68 degrees. Thus, light diffracted by one diffraction grating interferes with light diffracted by the other diffraction grating. In the case of the CD, in particular, intensity ratio of sub-beams by the +1st-order diffracted light and −1st-order diffracted light becomes large, so that error component of a tracking error signal also becomes larger.
In Japanese Laid-Open Patent Publication No. 2003-162831, in order to make amplitude of a push-pull signal of the sub-beams by the +1st-order diffracted light and −1st-order diffracted light substantially zero, the diffraction grating is formed of a phase-shifting diffraction grating so as to attain a phase difference to a part of the laser beam. This phase-shifting diffraction grating is formed so that a flat plate-like optical element shaped like a square, for example, is divided into four square regions and the grating groove in two regions which are symmetrical about the center of the optical element (optical axis) is shifted from the grating groove in the other two regions by ¼ pitch. However, it is difficult to produce the optical element having such complicated grating grooves with high accuracy. This may increase the production costs and generate new problems due to error in the shape of the grating grooves.
In Japanese Laid-Open Patent Publication No. 2004-39109, in order to prevent the laser beam for CD from being diffracted by the diffraction grating for DVD and the laser beam for DVD from being diffracted by the diffraction grating for CD, the diffraction grating is formed so that depth of the grating groove is made greater than the general diffraction grating and the duty ratio of the grating groove is shifted from 0.5. However, it is difficult to produce the optical element having such complicated grating grooves with high accuracy. This may increase the production costs and generate new problems due to error in the shape of the grating grooves.
In Japanese Laid-Open Patent Publication No. 2002-208166, both surfaces of an optical element are provided with a diffraction grating. One diffraction grating closer to a light source diffracts both of the laser beam for DVD and laser beam for CD and the other diffraction grating diffracts reflected light from an optical disc again, thereby guiding the laser beam for DVD and laser beam for CD to light receiving parts provided at respective different positions. With such a configuration, two pairs of light receiving devices are required, which runs counter to the object of reducing an optical pickup in size and weight and cutting the costs.